This invention relates generally to structure and method for effecting an adhesive bond through the use of induction heating and heat activatable adhesive.
The invention more specifically relates to a unitary, composite core structure of a U-shaped variety, capable of heating juxtaposed workpieces at differing rates.
A variety of systems are known for activating an adhesive layer between a pair of adherend surfaces through the use of application of heat. Perhaps the most desirable technique to activate such adhesive would be to subject the entire environment simultaneously to a heat source for a predetermined time duration. The time duration would preferably be the amount of time required to bring the adhesive to its desired temperature. Such a technique would heat each of the adherend surfaces as well as the adhesive which would promote an effective bond at each bonding surface. However, this type of total heating is often impractical and not desirable for a variety of reasons. The size and configuration of the parts to be heated often do not lend themselves to such a heating concept. Also, the total heating of a part may heat areas that are not intended to be heated and could conceivably damage certain areas of the part.
For these reasons, selective region heating has been utilized to produce a proper bond between two surfaces. Typical of such methods are induction heating methods using a variety of shapes and configurations of coil and core structures. One of the most advantageous and efficient coil and core structures utilized in the prior art incorporates a generally U-shaped core with an exciter winding placed thereabout so that the core can be positioned with the legs in direct contact with the workpiece to be heated. Upon application of current to the coil, a magnetic circuit is formed in the core and is completed through the workpiece. The core is structured so as to reduce heat looses within the core itself but to permit heat losses, resulting from eddy currents and hysteresis, to be isolated in the workpiece in a selected area located between the legs of the core. This type of a structure effectively heats a small area with little or no flux loss to the surrounding environment. In using such a system to activate heat activatable adhesive, the core is positioned in contact with, or in close proximity to, one of the two juxtaposed adherend surfaces so as to complete the magnetic circuit. This requires that the heat generated in the adherend surface be transferred to the adhesive positioned between the surfaces to properly activate and create a bond between the surfaces. Since adhesive is of an insulative material, quite often the proper heat does not reach the surface opposing the adherend surface which generates the heat. Since a proper bond requires a good "wetting" or bonding interface between the adhesive and both of the opposing adherend surfaces, it is important that the predetermined temperature level exist at both interface surfaces at substantially the same time.
Using such a single U-core system, one surface of the adhesive is either overheated or underheated if the other surface has obtained the proper heat.
Problems of this type are multiplied when workpieces of substantially different mass are to be adhered together since a greater amount of watts per pound will be required to bring the adherend surface of the larger mass to a proper temperature than would be to bring the adherend surface of the smaller mass to the proper temperature.